She was born in Paris in 1923, and ten years later as a child moved with her parents to New York City. She initially intended to be a concert pianist, but after obtaining her bachelor’s degree in Classical Archaeology in 1943 from Bryn Mawr College, she earned a master’s degree in archaeology and then her doctorate in Armenian, Byzantine and Medieval History (1958) at Columbia University.Garsoïan came to Columbia University in 1962 and became the first female professor to receive tenure at its Department of History. She was invited to Princeton University in 1977 to become the first female dean of its graduate school, but only stayed till 1979, when she returned to Columbia as the inaugural holder of the Gevork M. Avedissian Chair in Armenian History and Civilization.
She was one of the leading scholars in Armenian and Byzantine Studies, and part of the generation of scholars who integrated Armenology into American academia at the highest levels. As such, she became the first president of the Society for Armenian Studies (SAS) in 1975.
Garsoïan served as editor for many years of the Paris-based Revue des Études Arméniennes. She was a Fellow of the Medieval Academy of America and a Corresponding Fellow of the British Academy. She participated in the Byzantine Studies Symposium at Dumbarton Oaks, twice serving as a co-director.